The invention relates to a transmitter for wavelength division multiplexed signals.
Optical fiber telecommunication systems are undergoing continual expansion, fuelled by the need for more bandwidth. The nature of the expansion in demand for bandwidth is such that wavelength division multiplexing (WDM) of optical channels is required to overcome the bottleneck in capacity which arises in time-division-multiplexed (TDM), single-wavelength systems due to speed limitations of electronic circuits. State-of-the-art commercial systems use up to sixteen simultaneous channels to increase system capacity but the demand for capacity will continue to increase.
Although the capacity of the third telecommunications window is very large, loss limitations dictate that optical amplifiers are essential building blocks in modem networks and optical amplifiers, in particular erbium-doped fiber amplifiers (EDFA), determine the currently available practical bandwidth.
Current trends are towards dense wavelength division multiplexing with a 100 GHz channel spacing as the next generation of WDM comb standards.
For example, recent experiments have demonstrated 1 Tb/s transmission using 100×10 Gb/s as described in A. K. Srivastava et al., “1 Tb/s transmission of 100 WDM 10 Gb/s channels over 400 km of TrueWave™ fiber”, in OFC '98 Technical Digest, paper PD10, 1998.
Another example is the demonstration of transmission using 50×20 Gb/s WDM channels over 400 km and 600 km as described in S. Aisawa, T. Sakamoto, M. Fukui, J. Kani, M. Jinno and K. Oguchi, “Ultra-wide band, long distance WDM transmission demonstration: 1 Tb/s (50×20 Gb/s), 600 km transmission using 1550 and 1580 nm wavelength bands”, in OFC '98 Technical Digest, paper PD11, 1998.
There are two alternative approaches to implement high quality sources for telecommunication purposes, namely the semiconductor and the fiber distributed-feedback (DFB) laser. Commercial systems and most experimental systems being studied so far, for example the systems referred to in the above references, use various types of semiconductor lasers (SLs) as sources, for example DFB or DBR lasers. SLs are powered individually and are usually wavelength-stabilized (in order to meet the stringent telecom grid requirements) by the use of an external cavity. It is also well known that SLs are prone to aging effects and sudden failures resulting in a complete loss of the corresponding communication channel. This renders SLs quite unsuitable for integration, since failure of a single laser implies replacement of the entire integrated chip.
Fiber DFBs have been studied for the last few years since their first development at Southampton University, as reported in J. T. Kringlebotn, J.-L. Archambault, L. Reekie and D. N. Payne: ‘Er3+:Yb3+-codoped fiber distributed-feedback laser’, Optics Letters, 19(24), 2101–3, December 1994. Fiber DFB lasers have potential advantages in terms of wavelength setability and stability, as well as reliability, longevity and cost, such that they appear as a promising alternative to semiconductor laser sources. In addition and most importantly, there is no known degradation and failure mechanism for fiber or waveguide DFB lasers. Fiber and waveguide DFB lasers, though, still rely on high power semiconductor lasers for efficient pumping and large output powers.
In an article W. H. Loh, B. N. Samson, L. Dong, G. J. Cowle and K. Hsu, “High performance single frequency fiber grating-based erbium:ytterbium-codoped fiber lasers”, J. Lightwave Technology, vol. 16, no. 1, pp. 114–118 (1998) it is suggested that an array of semiconductor pump lasers can be used to power an array of fiber lasers. Specifically, in the above-referenced article there is proposed a 16-channel WDM transmitter in which the light outputs from all 16 pump lasers are combined and redistributed to 16 fiber lasers using a 16×16 splitter. In this case, each fiber laser is equally fed by all pump lasers and, therefore, failure of an individual pump laser will result in only a small reduction in the transmitter power of each channel supplied by the associated fiber laser.
The suggested architecture is limited to equal numbers of pump lasers and fiber lasers which may be undesirable for 16+ channel WDM transmitters in that an excessive number of pump lasers is needed. Moreover, it is generally undesirable to use a splitter in the suggested manner for large numbers of channels, since the power of all the pump lasers is localized in the active part of the splitter leading to undesirable non-linear effects. Furthermore, the suggestion in the article is limited to providing tolerance against pump failure, whereas a mechanism for positive failure recovery would be more desirable.